Interleukin-15 is a known T-cell growth factor that can support proliferation of an IL-2-dependent cell line, CTLL-2. IL-15 was first reported by Grabstein et al., in Science, 264:965 (1994) as a secreted cytokine comprising a 162-amino acid precursor polypeptide that contains a 48-amino acid leader sequence that results in a 114-amino acid mature protein. Grabstein et al. also describe the cloning of the full-length human cDNA encoding the 162-amino acid precursor, which contains a 316 bp 5' noncoding region and a 486 bp open reading frame (or a 489 bp open reading frame when including the 3 bp for the stop codon) and a 400 bp 3' noncoding region.
IL-15 shares many properties with IL-2. These properties include proliferation and activation of human and murine T cells, the induction of lymphokine activated killer cell (LAK) activity, natural killer cell (NK) activity, and cytotoxic T lymphocytes (CTL) activity, and costimulation of B cell proliferation and differentiation.
Additionally, IL-15 and IL-2 are structurally homologous molecules that are able to bind to at least three distinct receptor subunits on the T cell membrane surface. IL-2 receptors contain at least three subunits, .alpha., .beta. and .gamma. (Toshikazu et al., Science, 257:379 (1992)). Both IL-15 and IL-2 share binding to a common .beta.--.gamma. subunit complex, while each of IL-15 and IL-2 bind to a specific .alpha.-receptor subunit (IL-15R.alpha. and IL-2R.alpha., respectively). Recently, the IL-15R.alpha. was discovered and is the subject of copending application Ser. No. 08/300,903, now U.S. Pat. No. 5,591,630. Antibodies directed against the .alpha.-chain of the IL-2 receptor (anti-IL-2R.alpha.) have no effect on IL-15 binding (Grabstein et al., Id.). Antibodies directed against the .beta.-subunit of the IL-2 receptor, i.e., TU27, TU11, or Mik.beta.1, however, are able to block the activity of IL-15, suggesting that IL-15 uses the .beta.-subunit for signaling. Similarly, the .gamma.-chain of the IL-2 receptor is required for signal transduction (Giri et al., EMBO J., 13:2822 (1994)). The combination of the .beta. and the .gamma.-subunits of the IL-15 receptor complex, but neither subunit alone, bound IL-15 on transfected COS cells.
Certain disease states and physiological conditions are mediated by T cells. Such diseases include organ transplant rejection, graft versus host disease, autoimmune disease, rheumatoid arthritis, inflammatory bowel disease, dermatologic disorders, insulin-dependent diabetes mellitus, ocular disorders and idiopathic nephrotic syndrome/idiopathic membranous nephropathy. Indeed, allograft rejection and graft-versus-host disease (GVHD) have been associated with increased IL-2 receptor expression. T cells activated in response to foreign histocompatibility antigens appear to express the IL-2 receptor complex. Various therapies have been proposed and studied. For example, Tinubu et al. (J. Immunol., 153:4330 (1994)), reported that the anti-IL-2R.beta. monoclonal antibody, Mik.beta.1, prolongs primate cardiac allograft survival. There is an increase in IL-2R.beta.-subunit expression on CD4-and CD8-expressing cells in association with acute allograft rejection, which indicates that the IL-2R.beta.-subunit expression seems to increase on alloreactive T cells. See, for example, Niguma et al., Transplantation, 52:296 (1991).
However, prior to the present invention, there have been no therapies that focused on the IL-15 ligand-receptor interaction as a means of treating GVHD or in promoting allograft survival.